1. Field of the Invention
The present invention relates generally to processes for depositing thin films of metal oxides onto a variety of substrates. More particularly, the present invention involves the fabrication of such metal oxide films with high deposition rates using the activated reactive evaporation (ARE) technique.
2. Description of Related Art
A wide variety of simple evaporation processes have been used to deposit thin films of materials on various substrates. In these processes, vapors are produces from a source material by the application of heat or other energy source. The vapors produced from the source are then condensed onto a substrate. One problem experienced with such simple evaporation involves partial dissociation of the compounds during evaporation which results in less than full stoichiometry. This causes the formation of films which are dificient in one or more elements and therefore non-stoichiometric. Another problem involves those compounds which have a high melting point. A very high power density is required in order to obtain appreciable evaporation rates of these compounds. The application of such high power densities to billets fabricated by powder metallurgy methods may result in disintegration of the billet.
In response to the above problems, the process of reactive evaporation (RE) was developed. In reactive evaporation, metal atoms are evaporated from a thermally heated source in a vacuum chamber in the presence of a partial-pressure reactive gas, such as oxygen. The reactive gas and metal atoms react to form compounds which are deposited on the substrate. A modified form of reactive evaporation was developed in 1972 by R. F. Bunshah and A. C. Raghurham (J. Vac. Sci. Technol. 9, 1385 (1972)). The modified process involves introduction of a plasma between the evaporant source and the substrate. This modified evaporation process is commonly referred to as activated reactive evaporation (ARE). An advantage of ARE is that it enhances the probability of reactions taking place by activating or partially ionizing the evaporant atoms. In view of this advantage of the ARE technique, it would be desirable to extend the process to deposit high quality Al.sub.2 O.sub.3 films at very high deposition rates.
In any process for forming films on a substrate, it is desirable to maximize deposition rates while not adversely affecting film quality or compound stoichiometry. This is particularly true for aluminum oxide films which are used in many high technology applications. For example, aluminum oxide films have played an important role in various fields as an insulating layer in metal-insulator-semiconductor field effect transistor (MISFET), gate insulators in solid state hydrogen sensors, X-ray and accelerator neutralizers in nuclear reactors, tunnel barriers in Josephson tunnel junctions, antireflection coatings in solar cells, optical wave guides and protective layers for metal reflectors. In view of these applications, a variety of techniques have been used to synthesize Al.sub.2 O.sub.3 films. For example, techniques such as chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), glow discharge, electron beam evaporation of alumina, rf sputter deposition, reactive sputter deposition, ion beam sputter deposition and recently molecular layer epitaxy (MLE) have been used to synthesize aluminum oxide films.
Aluminum oxide films have been prepared by chemical vapor deposition utilizing aluminum chloride as the major reactant (S. K. Tung and R. E. Caffrey, Tran. Metall. Soc. AIME, 233 (1965) 572; A. S. Wong, G. M. Michal and I. E. Locci, J. Mater. Res., 3(5) (1988) 572). In this technique, a uniform deposit of aluminum oxide was produced by pyrohydrolizing aluminum chloride with hydrogen and carbon dioxide gas at approximately 850.degree. C. Aluminum oxide films have also been fabricated at lower substrate temperatures utilizing metal organic chemical vapor deposition techniques (J. Fournier, W. DeSisto, R. Brusasco, M. Sosnowski, R. Kershaw, J. Baglio, K. Dwight and A. Wold, Mat. Res. Bull., 23 (1988) 31; C. Dhanavantri and R. N. Karekar, Thin Solid Films, 169 (1989) 271). In these MOCVD processes, volatilized aluminum isoperoxide was thermally decomposed at substrate temperatures of between 350.degree. C. and 500.degree. C. to deposit the aluminum oxide films. In another technique, a plasma was employed to enhance the CVD process (Y. Catherine and A. Talebian, J. Electrochem. Soc., 17(2) (1988) 127). The plasma was produced by means of either a 450 kHz or 13.56 MHz discharge. A mixture composed of Al(CH.sub.3).sub.3 (TMA) with helium or argon as the carrier gas and CO.sub.2 was used as the reactant. The aluminum oxide films were prepared at substrate temperatures between 25.degree. C. and 350.degree. C.
High quality aluminum oxide films have also been prepared by evaporating aluminum oxide pellets in an environment of oxygen gas at substrate temperatures ranging from 25.degree. C. to 250.degree. C. (J. Saraie, S. Goto, Y. Kitao and Y. Yodoggawa, J. Electrochem. Soc. 134 (1987) 2805). Radio frequency sputter deposition and reactive sputter deposition have also been employed to produce aluminum films using aluminum oxide and aluminum metal targets, respectively. The ion beam sputter deposition technique utilized to deposit aluminum oxide films has used ionized argon gas which was accelerated and directed to the aluminum oxide target by an accelerator grid. The target material was sputtered off with oxygen gas being introduced to compensate for the partial dissociation of aluminum oxide during the sputtering process (C. Nishimura, K. Yanagisawa, A. Tago and T. Toshima, J. Vac. Sci. Technol., A5(3) (1987) 343; S. M. Arnold and B. E. Cole, Thin Solid Films, 165 (1988) 1).
With respect to molecular layer epitaxy, single crystals of alpha-aluminum oxide films have been produced (G. Oya, M. Yoshida and Y. Sawada, Appl. Phys. Lett., 51(15) (1987) 1143). In the MLE technique, an anhydrous aluminum chloride vapor and a helium/oxygen gas mixture were alternatively supplied to the substrate through separate pipes by opening and closing valves attached to the pipes. Aluminum oxide has been deposited by activated reactive evaporation wherein a one inch diameter billet of aluminum metal was evaporated from a rod-fed electron beam source in the presence of partial pressures of oxygen varying from 2.times.10.sup.-5 Torr to 2.times.10.sup.-4 Torr (R. F. Bunshah and R. J. Schramm, Thin Solids Film, 40 (1977) pp. 211-216).
Although high quality aluminum oxide films have been synthesized utilizing the above described techniques, the deposition rates obtained have been relatively low. For example, deposition rates of 4.6 to 33.9 nm per minute for MOCVD and 18 nm per minute for electron beam evaporation of aluminum oxide have been reported. Deposition rates for techniques such as ion beam sputter deposition and glow discharge are typically below 15 nm per minute. Aluminum oxide deposition rates as high as 90 nm per minute have been obtained using modified ion source geometry in an ion beam sputter deposition technique. Deposition rates for ARE have typically been well below 200 nm per minute.
An obstacle to increasing deposition rates of metal oxides, such as aluminum oxide, by the ARE technique is oxide poisoning of the metal billet or target material (i.e., formation of an oxide layer on top of the molten aluminum). Metal oxides, such as aluminum oxide, have a melting point which is much higher than the melting point of the metal. Accordingly, if a layer of aluminum oxide is allowed to form, the evaporation of metal from the source material is adversely affected. In the ARE technique, the oxygen which is introduced into the system to react with vaporized metal also tends to react with metal present on the billet surface. This leads to reduction of the deposition rate because of the lower evaporation rate of aluminum oxide as compared to pure aluminum. An increase in deposition rate can be achieved by increasing the electron beam current. However, this leads to deposition of substoichiometric metal rich films.
There is a continuing need to develop and define process conditions for the deposition of metal oxides by ARE wherein the deposition rate is increased without sacrificing film quality. It would be desirable to provide a process for depositing metal oxide films by ARE wherein fully stoichiometric, transparent films of metal oxides are deposited at rates on the order of 10 to 20 .mu.m/hour or higher.